1. Field of the Invention
The present general inventive concept relates to an autostereoscopic projection system, and more particularly, to an autostereoscopic projection system which can display a multi-view stereoscopic image using a single projection lens unit.
2. Description of the Related Art
An autostereoscopic display having a single projection lens and a single projection cathode ray tube (CRT) is disclosed in U.S. Pat. No. 5,132,839. In the referenced patent, stereoscopic images are displayed sequentially by simultaneously changing a direction of a projection beam. A disadvantage of the autostereoscopic display disclosed in the referenced patent is an influence of persistent image in a high frame rate image projector, and a bulky projection lens. A system employing only one projection panel to display all perspective views is disclosed in U.S. Pat. No. 5,430,474. However, the system of the referenced patent requires usage of more than one projection lens and corresponding illumination systems.
A stereo attachment for projectors and cameras is disclosed in U.S. Pat. No. 2,314,174. A disadvantage of the stereo attachment disclosed in U.S. Pat. No. 2,314,174, lies in a certain level of keystone distortion of images projected by the stereo attachment. Perspective images are not focused properly onto an entire screen area due to the keystone distortion. This disadvantage leads to more complicated problems if the same technique is applied in a compact stereoscopic or an autostereoscopic display employing a wide angle image projection.
FIG. 1 illustrates a conventional stereoscopic system employing a single projection lens in accordance with U.S. Pat. No. 2,314,174. The conventional stereoscopic system is used for stereoscopic cinema projection. Referring to FIG. 1, two adjacent perspective images recorded on a transparency 11 are simultaneously projected through a common projection lens 12 onto a screen 13. A mirrored prism 14, which is mounted near the projection lens 12, dissects a projection beam received from the projection lens 12 into two equal parts as partial beams, each of which is then reflected by a corresponding mirror 15 toward a corresponding polarizing filter 16. An angle of inclination of the mirrors 15 can be adjusted to converge image halves onto the screen 13. The two partial beams are perpendicularly polarized by the two corresponding polarizing filters 16. The two partial beams may be projected to a region 19 outside the screen 13 according to the angles of inclination of the corresponding mirrors 15.
If the corresponding mirrors 15 are parallel to facets of the prism 14, the two perspective images arriving at the region outside the screen 19 are focused without any distortion. However, the two perspective images are also separated from each other in a vertical direction when the corresponding mirrors 15 are parallel to the facets of the prism 14. For comfortable stereoscopic viewing, the vertical separation between the two perspective images should be minimized, which can be achieved by converging the two perspective images on a central part of the screen 13. To converge the two perspective images on the central part of the screen 13, the corresponding mirrors 15 should be declined at a certain angle, that is, about one-fourth of a projection angle. The projection angle is the angle at which the projection beam is projected by the projection lens 12. As a result, two images 17, which arrive at the screen 13, are unequally distorted and defocused everywhere except for one horizontal line on the central part of the screen 13.
The distortion of the two images 17 may be characterized as keystone distortion of different signs that produces undesirable perspective distortion of stereoscopic images. The greater the projection angle, the more distortion and defocusing that occurs in the two images 17. That is, the distortion and defocusing that occurs in compact types of stereoscopic and autostereoscopic displays like desktop, kiosk etc., which employ a wide projection angle is typically much greater than in cinema projection. Yet another disadvantage of the conventional stereoscopic system is that it cannot be used for a multi-view autostereoscopic application.
Additionally, stereoscopic images are typically viewed through polarizing eyeglasses 18. A similar beam splitting projection system that uses a special screen for the autostereoscopic projection may be employed instead of the polarizing eyeglasses 18. However, it is difficult to manufacture this beam splitting projection system used for the autostereoscopic projection.